The present invention relates generally to a telephone handset interface for a device having an audio input and, more specifically, to an interface for coupling a modem to a telephone instrument.
A modem is a telecommunications device that is used for interfacing a computer, facsimile machine or similar data device with a telephone line to enable the data device to communicate with another like device over the telephone line. A modem typically has a data connection to the computer or other data device and a two-wire RJ-11 telephone connector for connecting the modem to the telephone line.
Practitioners in the art have developed voice-over-data modems. In addition to the data connection and the telephone connection, a voice-over-data modem has a voice connection to a headset, a telephone-like handset, or other microphone/speaker pair. The modem allows a user to send or receive data over the telephone line while simultaneously speaking or listening to the other party over the same telephone line. The other party must have a compatible voice-over-data modem.
A microphone is typically designed to operate properly at a specific DC voltage level. Microphone elements of different headsets and telephone handsets, especially those produced by different manufacturers, may operate at different voltage levels. Therefore, a voice-over-data modem manufacturer may provide a handset or headset having a microphone element that is compatible with its modem. Nevertheless, the user may desire to use a different handset or headset. For example, the manufacturer may provide only a headset, and the user may desire to use a handset.
A modem, whether data-only or voice-over-data, cannot easily be connected to a PBX or office-type telephone system. The problem is described U.S. Pat. No. 4,907,267, issued to Gutzmer, entitled "MODEM INTERFACE DEVICE." In a PBX telephone system, multiple lines are serviced by a central controller. It is difficult to achieve dedicated line service for a modem because the telephone line to which a particular telephone instrument is connected carries line selection information and power in addition to the audio frequency communication signal of the handset. To solve this problem, the above-referenced U.S. patent describes a solution in which a modem is connected to the telephone system via the handset jack of the telephone instrument. The patent describes an interface device that allows a user to switch between voice and data communication. To use the interface device, both the modem and the handset that was unplugged from the telephone instrument are plugged into the device. The device is then plugged into the handset jack of the telephone instrument base. The device has a switch that allows the user to selectively connect either the handset or the modem to the handset jack. The interface device converts the two-wire modem connection to the four-wire handset connection using a transformer. The transformer also provides impedance matching.
Although a handset or headset can be connected directly to a voice-over-data modem, the voice-over-data feature does not obviate the need for a two-wire to four-wire conversion or the need for impedance matching to successfully interface the modem to a PBX telephone system via the handset jack of a telephone instrument base.
These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.